Required Practicals / OCR A / Practical 2
2 AS PAG 2

Determination of the Young modulus (PAG 2)

Determine the Young modulus of a metal wire by loading it and measuring extension.

Apparatus

  • Long metal wire (~2 m) clamped from above
  • Micrometer screw gauge
  • Metre rule and vernier scale
  • Slotted masses and hanger
  • Reference wire (same material, same clamp point)

Safety

  • Wear goggles in case the wire snaps.
  • Use a tray below the hanging masses.

Method

  1. Clamp both the reference wire and test wire from the same overhead support.
  2. Measure unstretched length L and diameter d (micrometer, three positions).
  3. Add masses to the test wire in 100 g increments; read extension x from vernier scale after each addition.
  4. Unload and confirm elastic recovery.
  5. Plot stress $\sigma = F/A$ vs strain $\varepsilon = x/L$; gradient $= E$.

Key Variables

Independent Applied force F
Dependent Extension x
Controlled Temperature; Length L; Cross-section A

Analysis and Results

  • $E = \sigma/\varepsilon = FL/(Ax)$.
  • Gradient of stress-strain graph $= E$.
  • Reference wire eliminates errors from support sag and thermal expansion.

Common Errors

  • Overloading the wire beyond its elastic limit.
  • Measuring diameter at only one point.
  • Not zeroing the vernier before the first mass is added.

Exam-style questions on this practical. Click Show mark scheme to reveal the answer after attempting each question.

Q1 4 marks

A steel wire (length 2.0 m, diameter 0.50 mm) extends by $1.4$ mm under a 20 N load. Calculate the Young modulus.

Q2 2 marks

Why is a reference wire used alongside the test wire?